Marsh grass masquerades as a native species

If you visit a saltmarsh on the East or Gulf coasts of the United States, you’ll see mostly smooth cordgrass, Spartina alterniflora. This grassy species forms dense colonies along the shoreline and provides a home for an array of native wildlife.

But in other parts of the world, such as Europe, China and the U.S. west coast, smooth cordgrass is an invasive species, and it can be a problem. In San Francisco Bay, for instance, the grass has colonized mudflats and competes with a local, native cordgrass. These changes in vegetation have affected the local bird population, including the endangered California clapper rail.

Smooth cordgrass can also be found growing along the Atlantic coast of South America, from Venezuela to Argentina. And there, it is considered a native species. But it shouldn’t be, Alejandro Bortolus of the Patagonian National Center in Chubut, Argentina, and colleagues arguein the November Diversity and Distributions.

If a species has been present in a landscape for a long time, determining whether it is native or not isn’t an easy task. This is especially true in South America, where there were relatively few scientists and naturalists over the enormous region. There aren’t records from native people there, and early European explorers only rarely brought along scientists. But since the patterns of where smooth cordgrass is found in South America don’t match those in North America, and because the species is known to be invasive in Europe and elsewhere, Bortolus and his colleagues thought that the grass might not be native to the region. They scoured historical and scientific reports and herbarium collections for evidence of when the plant might have arrived on the continent.

Early travelers to South America described seeing cordgrass or “roofing grass,” which the researchers believe was either needlegrass, dropseed or Spartina denisflora. This last species, a type of cordgass, was used by natives and Spaniards as roofing material. That is one reason that the team doesn’t think the early travelers were seeing smooth cordgrass. The other is that smooth cordgrass makes excellent forage for horses and cattle. If those travelers had come across smooth cordgrass, they would have noted it since good food for livestock was so important at that time.

Smooth cordgrass was first collected in Brazil in 1817, and at progressively later dates up and down the coast. It wasn’t recorded in Argentina until 1902 and in Venezuela until 1996. And it tends to be found near major harbors and ports, which suggests that the cordgrass arrived by ship. That’s probably how smooth cordgrass got to Europe in the 1800s, as dry packing material in a ship’s hold, and it may be how it got to South America.

The cordgrass may have been a rare species that has more recently expanded its South American range, the researchers acknowledge, but they don’t think that its population growth and patterns match that scenario well. Also, the smooth cordgrass found in South America doesn’t have the same tolerance for colder temperatures as the North American variety, and it only exhibits one of the three forms found in North America.

The team concludes that smooth cordgrass was probably introduced to South America from the southern region of the plant’s native North American range sometime before 1817. There may have been multiple introductions. (Genetic studies could clear this up, but they haven’t been done yet.) Once in South America, the plants colonized bare mudflats and turned them into grassy marshes. This would have caused habitat loss; a reduction in the diversity of fish, birds and invertebrates; and transformed food webs and physical processes in the ecosystem.

“In a period as short as 200 years,” the researchers write, “the invasion of South America by S. alterniflora has had catastrophic consequences, with this plant entirely reshaping coastal systems.”

South American marshes, therefore, may be only an “ecological mirage,” they argue, illusions that hamper our ability to perceive what is native and what is not. And we may be lured into thinking that ecosystems are pristine when, in fact, they are just another sign of humans’ long ecological reach.